I think I would be inclined to either (a) try to push the top closed a little more, if possible, to hold a de-spined GEM or (b) shim a de-spined GEM. I would not try to force the back of the razor to take a spined blade - this was not the original intention and I think going down that line would ruin the razor.
To shim, what you need to find is a piece of metal (or plastic) thin enough to slip over the blade so that the top clip holds the blade more securely.
Perhaps a piece of plastic from a pop bottle would do it? Install the blade into the razor and then push the little piece of plastic between the blade and the top cap to make it hold more better. If the principle works, neaten up the solution. Plastic would be a good start, since it's unlikely to cut you - thin metal can be had from drink cans, feeler blades, DE blades ... but care should be taken to minimise sharp edges.